Electromagnetic Current Meters Research Articles

Observations of Hydraulic Jumps in High-Energy Swash

Abstract The possible influence on swash-zone sediment transport of hydraulic jumps is investigated using field measurements from two natural beaches in high-energy (Hs = 1.5 to 3.7 meters) dissipative conditions. Results from measurements of cross-shore velocity using electromagnetic current meters at z = 3 centimeters, water depth using pressure transducers at z = 0, and cross-shore bore displacement using video time stacks suggest that hydraulic jumps may occur in approximately one-third of infragravity backwashes. Results from sediment load measurements using traps suggest that backwashes containing jumps can advect approximately 2.5 times as much sediment offshore than a backwash not containing a hydraulic jump. The overall effect of hydraulic jumps appears to be to enhance offshore transport, especially in high-energy conditions, when infragravity backwashes are more common.

Reynolds stress observations in continental shelf seas

The three basic observational approaches to estimating turbulence parameters in continental shelf seas are free-fall micro-structure probes from which dissipation is inferred; fast-sample (10–20 Hz) current meters in sea-bed frames measuring turbulence intensity directly; and fast-sample O(1 Hz) high-frequency Acoustic Doppler Current Profilers (ADCPs) where Reynolds stress profiles and hence turbulence production can be estimated from the variance of the along beam data. Uncertainties are associated with each approach since turbulence is a small-scale, high-frequency phenomenon and since estimates can easily be contaminated by the presence of surface waves. This paper concentrates on the latter two approaches, particularly the ADCP method, focussing on the degree of confidence that can be placed on the estimates. Results are presented from nine experiments from six sites in the North and Irish Seas and one in the Gulf of California, involving the deployment of 0.6 and 1.2 MHz standard broadband ADCPs mounted in sea-bed frames. The sites ranged from very tidally energetic, shallow (20 m deep) to low tidal energy, deeper (110 m). The ADCPs recorded data with a variety of sample regimes, from 2 to 0.5 Hz; bin sizes ranged from 0.25 to 1 m. In two of the experiments the ADCP near-bed Reynolds stress estimates were tested against independent estimates from toroidal electro-magnetic current meters measuring the three components of current (vertical and both horizontal) at 8 Hz, deployed on a nearby frame. In all cases the correlation coefficient squared between the two sets of Reynolds stress estimates was 0.7. In a further three recent deployments, an Acoustic Doppler Velocimeter (ADV) was deployed on the bottom frame with the ADV measuring volume located within the first ADCP bin and sampling at 20 or 25 Hz. The ADV measurements also show an explained variance of about 80% and a transfer function of about 1 during periods where waves were not present. One objective of these studies was to test and improve representation of dissipation processes in two- and three-dimensional numerical models, including the concept of the constant stress layer. At its very simplest, bottom stress is estimated from the depth-averaged flow via a quadratic drag law. Calculations from these measurements give values for the drag coefficient between 0.0006 and 0.0019, averaging 0.0011, smaller than the value used in most depth-averaged numerical models (0.0025). There is some evidence that the value of the drag coefficient is dependent on the tidal current speed.

Velocity and turbulence distribution around lotic macrophytes

Flow velocity and turbulence patterns were measured in and around a common lotic macrophyte, Ranunculus penicillatus subspecies pseudofluitans (stream water-crowfoot), using a two-dimensional electromagnetic current meter (EMCM). Due to the high shooting density of this species, there was a sharp velocity gradient at the plant boundary, with velocities dropping to a constantly low value after no more than 5 cm into the plant, thus forcing most of the flow over and around the macrophyte. There was a dead-water zone immediately downstream of the plant, beyond which the current moved in from the sides to allow flow under the trailing shoots. High turbulence intensities were recorded for both downstream and cross-stream velocity components at the lateral margins and downstream of the plant. Meanwhile, pulses of water upstream of the plant produced turbulence in the downstream component, but not in the cross-stream component.

Reach scale variability of turbulent flow characteristics in a gravel-bed river

In gravel-bed rivers, bed roughness is often controlled by the spatial distribution of pebble clusters and large protuberant clasts. The turbulent flow structure around these roughness elements has been studied extensively as opposed to the effects of the spatial variability of the bed roughness on the structure of the flow at the reach scale. In this paper, we examine the effects of roughness elements on the spatial variability of the mean flow velocity profiles and of the properties of turbulent flow structures at the reach scale. The experiment was carried out in two reaches of 25 m long in the Eaton North River (Québec). We produced detailed topographic maps of the river reaches to obtain the spatial distribution of the main roughness elements. The flow is characterized with velocity time series measured using electromagnetic current meters mounted on a vertical wading rod. We collected 65 velocity profiles from which we derived turbulent flow characteristics. Results showed that in spite of the complex configuration of the bed, velocity profiles are predominantly log linear. Mean and turbulent flow characteristics are only affected at a local scale and over short distances by the presence of roughness elements. At the reach scale, the distribution of individual roughness elements does not greatly control the spatial variability of turbulent flow properties.

高架橋波浪対策護岸における越波量および水塊打上げ高の現地観測

Field observations have been carried out to investigate on splash height and overtopping rate of block mound seawall at the Oyashirazu Coast in Niigata Prefecture. Splash heights were measured by using video camera system. Velocities and depths of a overtopping flow were directly measured by electro-magnetic current meter and ultrasonic-type elevation meter. The main results are in the followings: Relative heights of a splash depend on the relative crown height (hc/Ho) and deepwater wave steepness. They increase their values as the relative crown height become large, whereas the deepwater wave steepness become small. Standard deviations of splash height amount to about 25% of the mean value. The maximum velocities of overtopping flow reach from 6m/s to 8m/s with a depth of 5cm to 20cm. The peak values of overtopping rate measured amount to larger than 0.2m3/s/m.

PLUNGING AND STREAMING FLOWS IN POOL-AND-WEIR FISHWAYS

Fishways are river constructions which give a helping to fish migration. The rea re three types of flow formations in pool-and-weir fishways such as a plunging flow, streaming flow and intermixed flow of plunging and streaming flows. Rajaratnam et al. proposed a criterion formula that predicts the flow formations. However, this criterion formula has no physical meaning. Further the accuracy of prediction of their for mula is not high. In this study, the criterion formula, which can predict the flow formation in a pool-and-weir fishway, is derived semi-theoretically. The experiments were conducted with changing the aspect ratio and discharge in the pool-and-weir fishway. The water surface profiles were measured and also three components velocities were measured with a 3-D electromagnetic current meter. A new criterion formula, which is able to predict the flow formation, is proposed under the condition that the channel slope is 1 on 12.

INTERACTION BETWEEN FREE SURFACE OSCILLATION AND VELOCITY FLUCTUATIONS IN OPEN-CHANNEL FLOW WITH VEGETATION ZONE

Laboratory experiments were conducted using a two-component electromagnetic current-meter and a particle-image-velocimetry (PIV) to clarify the interaction between main flow and flexible artificial vegetation, which was attached to the central bottom of an open channel. In addition, interception plate 1mm thick was placed at the one-sided vegetation interface along the main flow direction.Results show that in the main flow region of the equilibrium state, horizontal components of the secondary currents take the directions toward the vegetation zone near the bottom and toward the non-vegetation zone near the surface. From the measurements, it was shown that this thin plate significantly suppressed water surface oscillations, horizontal periodical velocity variations and spanwise momentum transport from main flow region to vegetation zone.

1 min in the life of a river: selecting the optimal record length for the measurement of turbulence in fluvial boundary layers

In the analysis of instantaneous velocity measurements, it is important to consider carefully the properties of the instruments as well as the sampling frequency and the record length. Although several studies have examined the influence of instrument settings on velocity measurements in rivers, little information is available on the effects of the record length on these measurements and on the turbulence parameters that can be extracted from the time series. Decisions on the record length are often made ad hoc. Here, we quantify the effects of the sampling period on the precision of turbulence statistics (e.g., mean, standard deviation, skewness, Reynolds stress, quadrant analysis) obtained from velocity measurements in fluvial turbulent boundary layers. A bootstrapping method is used to determine the standard errors (S.E.) of turbulence statistics from 19 time series. The time series were sampled in three gravel-bed rivers using both Electromagnetic Current Meters (ECM) and Acoustic Doppler Velocimeters (ADV). The record lengths ranged between 20 and 60 min. The sampling frequencies were 20 and 25 Hz for the ECMs and ADVs, respectively. We show that, for most turbulence statistics, the optimal record length (minimum sampling effort to achieve low standard errors) ranges between 60 and 90 s. We discuss the implication of these record lengths for standard errors estimates and variance losses.

Dynamics of a reflective beach with a low tide terrace

Many beaches are characterised by having steep upper slopes, flat low-tide terraces, and mixed or bimodal sediment distributions. This study compares the surf zone hydrodynamic and sediment transport processes of these two dynamically different regimes on the beach at Teignmouth (UK) as part of the COAST3D project. In-situ measurements were made using pressure transducers, electromagnetic current meters and optical backscatter sensors simultaneously on the steep part of the beach (slope 1/10, D 50 1.1 mm) and on the terrace (slope 1/100, D 50 0.25 mm) during a range of incident wave conditions (H b 0.25–0.75 m). The steep upper beach was characterised by: plunging breakers, high wave height/water depth ( γ) values, large gravity band cross-shore velocity variance, a well-developed subharmonic, strong undertow, high suspended sediment concentrations, and sediment transport dominated by a strong mean offshore component. The shallow shelving terrace was characterised by spilling breakers, low γ values, weak undertow, gravity velocity variance decreasing shoreward, infragravity cross-shore velocity variance increasing shoreward, and stronger longshore currents than undertow. Mean suspended sediment concentrations on the terrace were an order of magnitude less than on the steep beach. The oscillatory cross-shore components of transport were onshore in the incident wave band but increasingly dominated by the infragravity band in progressively shallower water. Longshore sediment transport rates on the terrace were large compared to cross-shore transport rates. The measurements suggest that the dynamics of the terrace/steep beach exhibit characteristics of separate dissipative/reflective sites. However, interaction between the different regions does occur. In particular, the low tide terrace acts to reduce the wave height that is capable of reaching the steep beach, thereby protecting it to some degree from erosion.

The influence of bore turbulence on sediment transport in the swash and inner surf zones

The role of bore turbulence for sediment transport in the swash and inner surf zones is investigated using field measurements of (a) cross-shore velocity using an electromagnetic current meter at z=3 cm, (b) turbulent kinetic energy ( TKE) and cross-shore velocity using a 3-dimensional acoustic Döppler velocimeter at z=7.5 cm, (c) water depth using a pressure transducer at z=0, and (d) suspended sediment concentration ( SSC) using seven miniature optical backscatter sensors at z=1–32 cm, on a steep beach in high energy conditions ( H b ≈1.6 m). High SSC and high TKE values were associated with the passing of the bore front while the near-bed flow was undergoing rapid onshore acceleration. Peak TKE values tended to occur whilst the near-bed velocity was still offshore. The SSC peaked only slightly later than the TKE and therefore no evidence exists to support advection of pre-suspended sediment. Correlation tests are performed to assess the predictive capability of (a) a Bagnold-type model based on shear stress, (b) a similar model based on TKE, and (c) a model combining (a) and (b). When the data from the whole time series are used, the combined model improves r 2 values over the shear stress model by up to 9%. If only data within bore events are included, then the improvement increases to over 55%, suggesting a significant effect of TKE on sediment transport across the bore.

A sampler-based technique for the study of near-bed sediment dynamics: application in the eastern English Channel

A simple technique for studying near-bed hydrodynamics and sediment dynamics is presented. The method combines the use of (1) a benthic tripod hosting a series of electromagnetic current meters, and (2) a newly developed near-bed multi-level water-sediment mixture sampler. The instrument package was deployed successfully at a shallow water station in the eastern English Channel. The currents at elevation 0.9 m above the bottom were asymmetrical, the flood current peak being slightly stronger than the ebb current peak. At elevation 0.3 m above the bottom, the ebb/flood current peak asymmetry vanished. The observed SSCs (suspended sediment concentrations) were tidally modulated, with a contrasting vertical distribution over the ebb and flood phases of the tidal current: the profile was uniform in the ebb phase whereas a stratification appeared in the flood phase. The depth dependence and time evolution of the SSCs are attributable to a combination of local resuspension and advection-dispersion of remotely suspended fine sediment by ebb currents. Suspended sediment fluxes were uniform during the ebb phase and increased with elevation above the bottom during the flood phase.

LSPIV를 이용한 표면유속 측정기법의 검증 및 적용

본 연구에서는 LSPIV기법의 효율성과 적용성을 검증하기 위해 유속검정용 전동기계를 이용한 검증실험을 통하여 LSPIV기법을 검증하였다. 검증을 실시한 후에 LSPIV기법을 실제하천에 적용하였다 대상하천으로는 경안천의 지류인 능원천과 곤지암천을 선택하였으며, 3차원 유속계 및 기존의 표면유속 측정에 사용해 왔던 전자파 표면유속계의 측정결과와 비교하였다. 본 연구를 통해 나타난 결과를 살펴보면 검증실험에 사용된 전동차의 결과값과 비교했을 때 LSPIV기법의 측정값은 5％이내의 오차로 잘 일치하는 경향을 나타냈으며, 실제하천에 적용하여 기존의 측정장치와 비교하였을 때에도 최대 8％ 이내의 오차로 잘 일치하였다. The purpose of this study is to verify LSPIV technique. Verification was made using moving cart with known velocity. The difference of velocity values obtained using these methods are within 5％, which means that LSPIV can be used in the field with reasonable accuracy After verification, LSPIV was applied to the river. The Neungwon stream and the Gonjiam stream, tributaries of the Kyungan stream, were selected as the test sites for application. The results of the test application were compared with those obtained by 3-D electromagnetic current meter and electron-wave surface velocity meter. Results show that the velocity values obtained using LSPIV coincide well with those obtained using conventional devices with maximum difference of 8％.

Size, shape and dynamics of large-scale turbulent flow structures in a gravel-bed river

In this paper, we present a detailed investigation of the size, scale and dynamics of macro-turbulent flow structures in gravel-bed rivers. We used an array of seven electromagnetic current meters with high resolution in both space and time to measure the streamwise velocity fluctuations in a gravel-bed river. The array was deployed successively in various configurations in order to quantify the vertical, lateral and longitudinal extent of the flow structures and to estimate their advecting velocities. To depict the spatial and temporal properties of the flow structures, we used space–time velocity matrices, space–time correlation analysis and coherent-structure detection schemes. The results show that the large-scale turbulent flow structures in a gravel-bed river occupy the entire depth of the flow and that they are elongated and narrow. The length of the structures is 3 to 5 times the flow depth while the width is between 0.5 and 1 times flow depth. In spite of the high roughness of the bed, these values are similar to those reported in the literature for laboratory experiments on large-scale turbulent flow structures. The dynamics of the large-scale turbulent flow structures investigated using flow visualization highlight the interactions between the outer flow region and the near-bed region. Our evidence suggests that large-scale flow incursions trigger ejections in the near-bed region that can develop into megabursts that can reach the water surface.

DISCHARGE FORMULA OF SIDE-WEIR IN SUBCRITICAL OPEN-CHANNEL FLOWS

Severald ischargef ormulae, w hich can predict the dischargep er unit width in open-channefl lows with a sideweir, have been suggesteda s yet. The dischargec oefficientsi n such formulaew ereo btainede mpirically.I nthis study, the velocitym easurements in subcriticalo pen-channel flows were conducted with an electromagnetic current meter and also the flow depth measurements were conducted with a point gauge, with changing the inlet Frouden umber and the ratio between the inlet flow depth and the length of the side-weir. It was found that the direction of the velocity on the side-weir is almost constant in the vertical direction, but those changes in the streamwise direction. The relationship between the direction of such velocity and the inlet Frouden umber and the ratio between inlet flow depth and the length of the side-weirw as made clear. As a result, a new dischargef ormulao f side-weiri n subcritical open-channel flows with zero height was suggested.

EFFECTS OF SLOPE ATTACHED TO WEIR ON MIGRATION RATE IN POOLAND-WEIR FISHWAY

A slope with 45 degree against a horizontal line was attached on the first weir in a pool-and-weir fishway, which is located beside of a large weir in the Onga River, to allow fishes migration. The observation of migrating fishes at the fishway with a digital video camera and velocity measurements with a 3-D electromagnetic current meter were carried out after the improvement. It was found that the migration rate near the side wall is larger than that at the central region of the fishway, because instantaneous velocity near the side wall is smaller than that at the central region of the fishway, irrespective of that the time averaged velocity of both regions are almost same and also the eddy scale around the fish is larger than the body length of the migrating fishes.

STUDY ON LENGTH OF HYDRAULIC JUMP WITH ABRUPT RISE

Relationship between the upstream flow depth and downstream one of a hydraulic jump with and without abrupt rise is derived from the continuity equation and momentum equation. On the other hand, such theoretical equations cannot give some information on the streamwise length of the hydraulic jump. In this study, the length of the hydraulic jump with abrupt rise was measured with changing both of the inlet Froude number and the ratio between the inlet flow depth and the height of abrupt rise by making use of a point gauge. The velocity measurements in and around the hydraulic jump were also conducted with two-components type of electromagnetic current meters. It was found that the length of the hydraulic jump with abrupt rise is affected both of the inlet Froude number and the ratio between the inlet flow depth and the height of abrupt rise. As a result, a new formula about the length of the hydraulic jump with abrupt rise is suggested.

An optical technique for the measurement of longshore currents

We present an optical method (optical current meter) to measure the longshore component of nearshore surface currents by measuring the alongshore drift of persistent sea foam in the surf zone. The method uses short time series of video data collected from an alongshore array of pixels. These space‐time data are first Fourier transformed to a frequency‐wave number spectrum and, finally, to a velocity spectrum. A model of the velocity spectrum is fit to the observed spectrum to estimate the foam drift velocity. Confidence intervals and other measures of the input and output data quality are calculated. Field test comparisons were made against an in situ bidirectional electromagnetic current meter on the basis of 1 month of video data from the 1997 Sandy Duck field experiment. The root mean square error between the two approaches was 0.10 m/s. Linear regression analysis showed the gain between the two instruments to not be statistically different from one. Differences between the surface and interior measurements were compared to forcing mechanisms that may cause surface velocity shear. Velocity offsets and alongshore wind stress were well correlated for cases when waves and wind were not aligned to within ±45°, when wind‐ and wave‐forced currents are reasonably separable. Calculated wind‐dependent surface current shear, modeled as a surface boundary layer, correlated well with the observed velocity offsets for observations of nonalignment between wind and waves. This technique can be applied to study large‐scale coastal behavior.

IMPROVEMENT MEASURES OF POOL-AND-WEIR FISHWAY WITH LARGE WATER DIFFERENCE

A pool-and-weir fishway has been constructed at one side of the weir which stands across a river for irrigation. Unfortunately, the deference of water elevation between the upper stream and the first pool of this fishway is relatively large in comparison with that between another pools. Therefore, the velocity of the falling flow is higher than the burst speed of the small fishes. As a result, such fishes cannot migrate to the upstream of the river. Several improvements for this fishway are suggested in this study. The flow velocity and the water surface fluctuations in such improved fishways were measured with a 3-D electromagnetic current meter and a water-wave gauge. It was verified that the falling velocity in one of improved fishways is lower than the burst speed of the small fishes.

７．流体力学 プールタイプ魚道におけるオイカワの跳躍遡上と水理特性との関係

A pool-and-weir fishway was constructed beside of a large weir in the river, which is located at the northern part of Kyushu Island. In this fishway, all migrating fishes do not swim in the water flow running above the weir but are jumping in the air. Behaviors of jumping fishes whose name is Zakko Platypus are recorded by a digital video camera and three components of velocity in the first pool were measured with a 3-D electromagnetic current meter. It was found that almost fishes were jumping near the side-wall due to the velocity of the falling flow running above the weir near the side-wall is slower than that at the center of the channel. The initial speed of the migrating fishes is in proportion to the body length of migrating fishes.

遡上域の漂砂濃度測定手法の検討

To measure the suspended sediment concentration in the swash zone of real beach, a concentration meter using the infrared backscatter method, a CCD camera, and an electro-magnetic current meter were employed. Clear peaks were appeared in the time series of the suspended sediment concentration. Averaged values of these peaks were similar to the values from sediment transport rates measured simultaneously by sand traps. It was assumed that the occurrence of the concentration peak was depended on the offshore current velocity on the eve of it rather than the current velocity at the sametime. From cross spectrum analysis, suspended sediment concentration was correlated well with the on-offshore standing wave.